Switch cabinets or similar apparatus that contains electrical and/or electronic components or sub-assemblies are as a rule air-conditioned or actively ventilated. Such enforced ventilation may be necessitated not only by thermal emission from the components themselves but by an unfavorable position of the cabinet, for instance in the vicinity of heat-emitting machines or in surroundings that are too cool for the components or sub-assemblies, because without such ventilation a climate develops inside the cabinet that would endanger the function of the components.
Monitoring of the air current is customarily achieved by monitoring the current supplied to the fan. On one hand this requires relatively elaborate switching technology, while on the other hand it is not very reliable, because it is possible for even a fan supplied with the correct current to be unable to generate a sufficient air current, e.g. when a filter has been inserted.